Is It Possible To Overcharge The Traction Battery Via Plug-In Mode?

No problem with leaving it plugged in. The car will turn it off when ready.

Extension cords may not pull electricity correctly. They can be used but need to be heavy duty cords. Best not to use them unless a last resort.

 

The problem with using an extension cord is if it isn't at least 14 gauge there will too much voltage drop in it. This makes the charger draw higher current, which can cause overheating. If you have to use an extension cord get one that is 12 gauge and no longer than 50 feet. This would give you similar voltage drop to a properly wired outlet. If you have to, a 12 gauge 100 ft cord can be used, but be sure with any of them to get a good quality one. The ends can also cause voltage drop on poor quality cords. Remember, you are not powering just a light bulb, the Prius is a medium to heavy current draw, and the cord must be chosen accordingly.

 

Maggie, you'll see that the charging cable connector has five pins. Two are for the electric power. Two others are for communication signals between the charger in the car and the power supply in the lump in the cable. The 5th is a proximity pin that tells the system when the connector is plugged in or pulled out.

The system is smart. The parts communicate and automatically do what you've been doing manually. Leave it plugged in and sleep well.